1. Field of the Invention
The field of the invention is phantoms used to simulate the human body during a radiologic examination. In particular, the field of the invention is phantoms used to simulate radiologic imaging during an X-ray examination in which a high contrast substance is introduced into the patient.
2. Background Art
X-rays are short wavelength electromagnetic waves of 10.sup.-3 to 10 nm. These waves can be produced when high speed electrons strike a solid target. Because X-rays penetrate materials that are opaque to light, such as the human body, and may be detected photographically, they are of great use in medical diagnostics. In a typical X-ray examination, the patient is positioned between the X-ray emitting device and a piece of X-ray-sensitive film. As the X-rays encounter internal organs and bones, they will be absorbed, scattered, or transmitted. The transmitted X-rays can register an image on the film. In this manner, an image of the patient's internal organs can be obtained.
In more advanced X-ray examination systems, the image obtained from the X-ray bombardment is translated into a fluorographic image that may be viewed on a TV screen. This image translation is an advantage during a prolonged exam when many images must be viewed or when the purpose of the exam is to visualize the organs working together. We call this image translation the "image acquisition system."
X-ray phantoms are devices used to simulate the human body during an X-ray examination. A phantom is usually composed of material that mimics human tissue in its ability to produce absorption and scattering of radiation. This ability to absorb and scatter radiation is expressed by the attenuation coefficient, which is a function of chemical elements of which the material is composed and the spectrum of energies used in the examination. Variations of attenuation coefficients and thicknesses among materials produce contrast in an X-ray image. Two substances with the same attenuation coefficient and thickness will similarly absorb and scatter X-rays under given imaging conditions and will produce the same contrast with respect to a third substance during an X-ray examination.
We define the "contrast ratio" of a contrast element as the quotient of the quantity of X-rays transmitted by a reference material to the quantity of X-rays transmitted by the contrast element for specified imaging conditions. When numerical values of contrast ratio are stated herein, they refer to the following imaging conditions:
(a) The X-ray emitting device provides a bremsstrahlung spectrum with a maximum energy of 80 keV and filtered by 0.2 mm copper in addition to the inherent filtration of the X-ray tube, housing, and collimator,
(b) the ratio is calculated in terms of the quantity (mR/mAs) of primary (unscattered) radiation exiting the phantom; exposure due to scattered radiation is subtracted.
Simulations using an X-ray phantom can be useful in calibrating the dosage to be used during an X-ray procedure. X-ray phantoms are also useful in calibrating the image acquisition system that converts the X-ray image into a fluorographic image that may be visualized on a screen during the X-ray exam. By using a phantom, the X-ray technician obtains reproducible imaging conditions and avoids exposing a patient to unnecessary radiation.
One example of an exam in which a phantom would be useful is a voiding cystourethrogram (VCUG). In this diagnostic exam, a patient's bladder is filled with an iodine-containing compound. Because iodine has a very high attenuation coefficient, the contrast ratio of the iodine-filled bladder and urethra will be high with respect to the organs and tissues surrounding them. The bladder, the urethra and other elements of the urinary system will become visible to the diagnostician through a fluoroscopic image obtained from the original X-ray image. A VCUG is often performed on patients to diagnose abnormalities of the bladder and urethra. A VCUG exam is simply one example of an X-ray examination procedure in which a high contrast substance is introduced into the patient.
VCUG exams present specific radiologic imaging problems. Existing phantoms fail to simulate the high contrast between the patient's iodine-filled bladder and the patient's soft tissues and bones. This failure is especially troublesome with pediatric patients who have a small body mass and less dense bones. In these pediatric patients, the contrasts between the iodine-filled bladder and these low attenuation value elements is overwhelming.
An additional problem with existing phantoms used in VCUG exams concerns the failure of the phantom to mimic the size and positioning of the patient's bladder in the X-ray field. This failure results in an over-exposed image for the following reason: During the VCUG exam, a large quantity of iodine-containing contrast medium is introduced into the patient's urinary bladder, and the patient's bladder is frequently positioned in the center of the X-ray image. In most X-ray examination mechanisms, a sensing window of the automatic exposure control (AEC) that controls the quantity of radiation used to acquire the image senses the radiation in the center of the X-ray image. Positioning the patient's bladder in the center of the X-ray screen places the bladder in the center of this sensing window. Since the iodine-containing bladder transmits very little X-radiation, the image in that region is very dark. If not calibrated to account for the low X-ray transmission in the bladder region, the AEC reacts by causing an over-exposure of the portion of the image that surrounds the bladder.
What is needed in the art of X-ray phantoms is a phantom capable of simulating a patient during an X-ray examination that involves introducing a high contrast substance into the patient.